First of all. 4.6 is not a mild overclock. With air coolers is really hard to hit the 4.6, since the vcore you could apply is a little bit lower than the one of an AIO.
For an X99 build stay away from BEQUIET. They have one of the worst implementation. Although for the 'consumer' level products works great, for the x99 is a complete trouble to install the cooler. Also it has a relatively low ram tolerance. Which could be quite problematic.
My recommendations for an X99 air cooled are (from best to worst IMO): Noctua D15s, D15, Cryorig R1, Thermal Right (IBE Extreme). Also the new Cooler Master (Air Maker I believe) has some great results with debatable aesthetics and a very very premium price.
If you want a single tower you should check the ThermalRIght Macho Hr-02 or the Grand Macho.